You choose whether to put +1/+1 counters on the creature or create Servo tokens as the fabricate ability is resolving. No player may take actions between the time you choose and the time that counters are added or tokens are created.

9/20/2016

Fabricate doesn’t cause the creature with the ability to enter the battlefield with +1/+1 counters already on it. For example, Weaponcraft Enthusiast will enter the battlefield as a 0/1 creature, then its fabricate ability goes on the stack. Players may take actions (such as casting instants) while the ability is waiting to resolve.

9/20/2016

If you can’t put +1/+1 counters on the creature for any reason as fabricate resolves (for instance, if it’s no longer on the battlefield), you just create Servo tokens.